Radiation from a diode-laser or an array thereof is now extensively used for optical pumping of lasers having a solid-state gain-medium, in particular lasers wherein the wherein the gain-medium is neodymium-doped yttrium aluminum garnet (Nd:YAG) or neodymium-doped yttrium ortho-vanadate (Nd:YVO4). Diode-laser radiation can be generated with efficiencies up to about 50% or greater. The wavelength of the radiation (emission-wavelength) can be selected, depending on the composition of the diode-laser materials, to match the peak-absorption wavelengths of the gain-media. The bandwidth of the radiation is comparable with the FWHM bandwidth of absorption peaks in the absorption spectrum of the gain-media.
A single diode-laser does not provide adequate power for pumping a high-power-solid state laser. A two-dimensional array of diode-lasers is required to pump really high-power lasers, for example, lasers having a peak power of about 200 W or greater. Such lasers are typically operated in a repetitively pulsed manner by driving the diode-laser array in a repetitively pulsed manner.
A two-dimensional array of diode-lasers is typically made by stacking one-dimensional diode-laser arrays known in the art as diode-laser bars. A diode-laser bar includes a plurality of individual diode-lasers (emitters) formed in semiconductor layers epitaxially-grown on a single elongated substrate. The substrate or bar (cut from a disk substrate) typically has a length of about 1 centimeter (cm), a width of about 1 millimeter (mm), and a height of about 140 micrometers (μm) or less. The emitters emit along a propagation-axis in the width direction of the bar, and have slow-axis (low beam-divergence) in the length direction of the bar and a fast-axis (high beam-divergence) perpendicular to the slow axis, i.e., in the height direction of the bar. Current is typically passed through the bars electrically connected in series, with emitters in any one bar connected in parallel
One drawback of diode-lasers is that the emission wavelength of a diode-laser or diode-laser bar is relatively strongly dependent on the diode-laser temperature. By way of example, for GaAs P/InGaAs diode-lasers, peak emission wavelength varies by about 0.3 nanometers (nm) per ° C. The diode-laser bar temperature, absent effective cooling, depends, inter alia, on the current passed through the diode-laser bar and the pulse-duration with which the diode-laser bar is driven. In a diode-laser bar stack (two-dimensional diode-laser array) the total emission from the stack is brighter the closer together the one dimensional-emitter arrays of the bars are in the stack.
With a close-stacking, providing equal cooling of the bars is extremely difficult if not impossible, as only outermost bars of the stack can be contacted by massive cooling members. This means that bars in the center of a stack will get hotter than bars at or near the top or bottom of the stack making wavelength control of individual bars very difficult.
In U.S. Patent Application Publication No. 2010/0183039, assigned to the assignee of the present invention, and the complete disclosure of which is hereby incorporated herein by reference, a diode-laser bar stack is described in which diode-laser bars are selected with different emission wavelengths at the same temperature and located in the stack such that at a nominal operating condition of the stack, where the bars reach different temperatures, the total emission of the stack has a bandwidth significantly greater than that of any one bar in the stack. In this way, it can be arranged that the absorption peak of a gain-medium being pumped can lie within the total emission bandwidth at any anticipated range of operating conditions (temperatures) of the stack. This eliminates a need to control the stack temperature by active means.
FIG. 1 schematically illustrates a prior-art diode-laser pumping arrangement 10. The arrangement is described in detail in the above-referenced patent publication. The arrangement includes a stack 11 of six diode-laser bars 12A, 12B, 12C, 12D, 12E, and 12F. Each bar includes a heterostructure 14 grown on a substrate 16. Diode-laser emitters (not shown) are designated within the heterostructure, as is known in the art. The fast-axis, slow-axis, and emission-direction (propagation-axis) are indicated in FIG. 1 by axes Y, X, and Z, respectively. The bars are soldered one to the next, with the epitaxial-layer side of one bar soldered to the substrate side of an adjacent bar such that the emitters are connected in series-parallel.
Stack 11 is sandwiched between a heat-sink member 18 and a heat-sink member 20, with both heat-sink members being supported on a base 22. There is a space 24 between the stack and the base. The epitaxial side 14F of bar 12F is in thermal contact with heat-sink member 20. The substrate side 16A of bar 12A is in thermal contact with heat-sink member 18. The diode-laser bars are in thermal contact with each other, with the epitaxial side of one in thermal contact with the substrate side of the next except of bar 12F. Heat-sink members 18 and 20 are insulated from base 22 by insulating layers 17 and 19 respectively. Current from a pulsed power supply (not shown) for driving the stack is connected to the stack by attaching a positive lead to heat-sink member 20 and a negative lead to heat-sink member 18.
FIG. 2 is a graph schematically illustrating the calculated total emission spectrum (solid curve) of the stack of FIG. 1 overlaid with the absorption spectrum (dashed curve) of Nd:YAG. It is assumed that the stack is driven in a pulsed mode with a pulse duration of 250 microseconds (μs), a pulse-repetition frequency (PRF) of 2 Hertz (Hz) and an average power of 200 Watts (w) per bar. It is assumed that the nominal emission-wavelengths of bars 12A, 12B, 12C, 12D, 12E, and 12F are 801.71 nm, 808.10 nm, 804.49 nm, 808.10 nm, 804.43 nm, and 801.05 nm, respectively.
The nominal bandwidth of the total emission from the stack is about 10.0 nm, with the center wavelength close to the 808 nm absorption peak of the Nd:YAG absorption spectrum. In theory at least, the average temperature of the six bars could vary by about ±10° C. from the temperature providing the emission spectrum of FIG. 2, with the 808 nm absorption peak still remaining within the total emission bandwidth.
A particular drawback of this prior-art stack arrangement is that only a fraction of the total emission power provided by the stack (about 25% in the overlay of FIG. 1) is absorbed by the gain-medium. This considerably reduces the electrical to optical pumping efficiency of the stack. There is a need to provide a diode-laser bar stack for optical pumping that is capable of operating over a relatively wide uncontrolled temperature range but will deliver radiation in only a relatively narrow wavelength range around a gain-medium absorption peak of interest.